Solid-state foaming of a thermoplastic material is a process for generating a microstructure in the material that includes many small bubbles or voids. The process includes exposing the material to an atmosphere of a gas at an elevated pressure for a period of time to infuse the gas into the material. After the material has absorbed enough gas, the material is exposed to an atmosphere having less pressure, and is heated, but not melted, to cause the gas in the material to nucleate bubbles. When the bubbles have grown to a desired size, or when a specific amount of bubbles have nucleated, the material is cooled. Because the material remains a solid (does not melt) during the whole process, the material is foamed in the solid state.
Because, it takes time for enough gas in the pressurized atmosphere to infuse the thermoplastic material, the solid-state foaming process is typically done as a batch process—that is, not as a continuous process. To maximize the amount of material that can be processed during a single batch, the thermoplastic material is formed into a sheet and wound onto a roll (10 in FIGS. 1A and 1B). To promote gas infusion into all layers of the roll 10, and thus reduce the period of time that the thermoplastic material is exposed to the gas, a sheet of a gas-permeable material is interleaved between each layer of the thermoplastic material in the roll 10. The gas-permeable material promotes gas infusion throughout the whole, rolled-up thermoplastic material by keeping each of the layers of the roll 10 separated and by allowing easy passage of the gas throughout the gas-permeable sheet's microstructure to expose all of the rolled-up thermoplastic polymer material.
Rolls of thermoplastic material interleaved with gas-permeable material are typically large in diameter and weigh several hundred pounds. Because of their size and weight, such rolls are formed in a horizontal position as shown in FIG. 1A. In the horizontal position, one can more securely support and maneuver the roll 10 by holding onto the hub 12 because the weight of the roll 10 is supported at both ends of 14a and 14b of the hub 12.
Unfortunately, forming, processing and unwinding the roll 10 in a horizontal position can cause problems. When formed in the horizontal position, the roll 10 can become tightly wound—i.e. the individual layers of thermoplastic material and gas-permeable material that make up the roll 10 are compressed against adjacent layers. This reduces the gas permeability of the gas-permeable layers of the roll 10, and thus hinders the flow of gas throughout the gas-permeable layer during the roll's exposure to an atmosphere of pressurized gas. Consequently, some regions of the thermoplastic material do not absorb enough gas which causes an uneven distribution of bubbles in the sheet's microstructure when the sheet is subsequently foamed.
When the roll 10 is gas-infused in the horizontal position, the roll 10 can obtain an uneven distribution of gas in the material because as the thermoplastic material in the roll absorbs the gas the thermoplastic material gets heavier and sags (shown in FIG. 1B). This in turn causes the top portion 16 of the roll 10 to compress, which, as discussed above, reduces the permeability of the gas-permeable layers located in the compressed region and can cause an uneven distribution of bubbles in the sheet's microstructure when the sheet is subsequently foamed.
When the roll 10 is unwound in the horizontal position, the thermoplastic material can experience a fluctuation in tension. If the roll 10 becomes unbalanced, then as the heavier region 18 travels up during the roll's rotation, one must exert more force on the layer being pulled off the roll 10 to counter the heavier region's resistance to the roll's rotation. And, as the heavier region 18 travels down during the roll's rotation, one must exert less force on the layer being pulled off the roll 10 to counter the heavier region 18 urging the roll 10 to rotate faster. This fluctuation in the tension of the thermoplastic polymer material can cause the material to warp as the material is foamed.